Sewage purification is performed by 
A. Microbes
B. Fertilisers
C. Antibiotics
D. Antiseptics

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Hint: Sewage is created by domestic and industrial establishments. It contains household waste liquid from baths, toilets, showers, sinks, kitchens, and so forth that is disposed of via sewers. In several areas, sewage also contains liquid waste from industry and commerce.
Complete Step-by-step answer:
The cleaning of polluted water by eliminating pollutant before it is discharged into water bodies. Such a process of wastewater treatment is referred to as sewage treatment.
Sewage treatment is carried out in three stages: primary, secondary, and tertiary treatment. The secondary treatment eliminates dissolved and suspended biological matter, using microorganisms in a controlled environment. Most secondary treatment systems use aerobic bacteria, which consume the organic components of the sewage (fat, sugar, and so on).
Some systems use fixed film systems, in which the bacteria grow on filters, and the water passes through them. Suspended growth systems use activated sludge, where decomposing bacteria are mixed directly into the sewage. Because oxygen is critical to bacterial growth, the sewage is often mixed with air to make it easier for decomposition.

Therefore the correct answer is Option A.
Note: Human health and the environment are mainly influenced by the direct disposal of industrial and human effluents into natural resources without any treatment.  Sewage treatment is essential to decrease the toxicity of sewage and maintain a safe and healthy environment, and also promote human welfare. Sewage contains an enormous number of organic matters which are toxic. Microorganisms are widely used in sewage treatment plants in order to remove this toxic organic matter.